1. Field of the Invention
This invention relates to an autofocusing device for video cameras and more particularly, to an autofocusing method of a passive type which allows autofocusing even though an object is blurred or located in a dark place.
2. Description of the Prior Art
Generally, there are two types of autofocusing devices in the art. One is an active type, in which an infrared or ultrasonic emitter and receiver are mounted on a video camera, the distance between the camera and object is measured by means of infrared rays or ultrasonic waves emitted, reflected and received, and autofocusing is accomplished by driving a focusing lens motor based on the measured distance. The other is a passive type, in which a focusing lens is adjusted to the position of the maximum contrast by utilizing a contrast signal extracted from an image output signal obtained by an image pick-up element.
The passive type, unlike the active type, consumes little power since it does not need to emit the infrared rays or ultrasonic waves of sufficient strength, has no parallax error by analyzing and processing the image output signal, accomplishes accurate autofocusing when the objects are far apart from the camera, when the objects are slanted or inclined, and when the objects are located outside of windows, has a compact size since it does not need any external units such as infrared or ultrasonic emitters and receivers when compared with the active type.
Thus, since the passive type has the advantages as described above, it is more popular recently. The passive typed autofocusing technologies are disclosed in Japanese patent Laid-Open No. 88-59274 entitled "autofocusing device", Japanese patent Laid-Open No. 88-59275 entitled "autofocusing device", Japanese patent Laid-Open No. 87-112112 entitled "autofocusing device", Japanese patent Laid-Open No. 88-20973 entitled "autofocusing device", and Sanyo Technical Review VOL. 17 NO. 2, August 1985. One of a conventional passive typed autofocusing device is schematically illustrated in FIG. 1. As shown in FIG. 1, signals obtained from the video signals(Y signal) which lie only within focus range are applied to a band-pass filter, an output signal of which is then applied to a detecting circuit which produces a direct output signal, which is proportional to a contrast and DC level is converted to a digital signal by an analog-to-digital convertor (AD converter). The digital signal which is proportional to the contrast is integrated for one frame by a digital integrating circuit, the output of which is applied to a focusing circuit as a focusing control signal. The focusing circuit controls a focusing lens driving motor until the maximum contrast is obtained thereby adjusting the focus. However, in such a passive type video camera, since the difference between the contrast when in focus and the one when not in focus is slight when the contrast of blurred object is low or when the object is located in a dark place, it is sometimes difficult to adjust the focus and to shoot the objects. Furthermore, there is a possibility of driving of the lens motor in the wrong direction because, in addition to an original signal, the high frequency components arising from quantizing noise or external noise also function as high frequency contrast components. Such problems can not be solved by the conventional passive type device and method.